This invention relates to an elastic fluorohydrocarbon resin and a polymerization method for producing the same.
There are various fluorine-containing crystalline polymers which serve as synthetic resins and have wide application by reason of their characteristic properties attributed to the presence of C--F bond, such as good heat resistance, high resistance to oils and many other chemicals and good weather resistance. However, these crystalline resins are unsuited to some uses wherein elasticity or flexibility of the employed resin is a matter of importance, as in the cases of pipes, gaskets, sealing elements, etc.
Where elasticity or flexibility is required besides the favorable properties of fluorohydrocarbon resins it is usual to use fluoroelastomer. However, fluoroelastomers fully exhibit their characteristic physical properties and particularly dynamic properties only after completion of a cross-linking process including the steps of kneading the fluoroelastomer in unvalcanized state with the addition of a cross-linking agent, fillers and stabilizers and subjecting the kneaded rubber to a heat treatment to accomplish cross-linking. It is inevitable, therefore, that molding of fluoroelastomer requires more complicated operations than molding of crystalline fluorohydrocarbon resins, and often restrictions are placed on the shapes of the fluoroelastomer articles. Besides, fluoroelastomers after the cross-linking process can hardly be remelted for the purpose of additional processing or working.
To obtain a fluorine-containing resin which has sufficient elasticity but does not need any cross-linking treatment, it has been tried to copolymerize a fluorine-containing monomer that is capable of providing a crystalline polymer having a relatively low glass transition temperature T.sub.g with a different monomer that is capable of sufficiently lowering the degree of crystallinity of the resultant copolymer. However, copolymers obtained by this method generally become lower in melting temperature and, hence, in the upper boundary of the temperature ranges in which the respective copolymers are practicable. Besides, the copolymers tend to undergo considerable changes in the modulus of elasticity with changes in temperature within the aforementioned ranges.
Also it has been tried to obtain a desirable resin by blending a crystalline fluorohydrocarbon resin with either a plasticizer or an elastic polymer. In practice, however, not so many kinds of plasticizers and polymers are known as sufficiently high in mutual solubility with crystalline fluorohydrocarbon polymers. Even when a blending material relatively high in the mutual solubility can be used, often it is impossible to use a desirably large amount of the blending material without adversely influencing the properties of the blended resin compositions. Furthermore, the blended resin compositions are liable to locally remain in a heterogeneously mixed state and, therefore, fail to fully exhibit the expected physical properties.
Among crystalline fluorohydrocarbon resins, polyvinylidene fluoride (will be abbreviated to PVDF) is well known as very stable even to highly corrosive chemicals, to ultraviolet rays and to radioactive rays, excellent in mechanical and electrical properties, and superior in workability to other fluorohydrocarbon resins. Accordingly, PVDF has wide applications and is largely used for laminating or coating metallic materials and for covering electric wires and cables.
However, PVDF lacks elasticity by reason of high degree of crystallinity and therefore has some shortcomings. For example, PVDF coverings on electric wires tend to spontaneously crack during storage of the covered wires, and PVDF sheets formed by extrusion or drawing are liable to become relatively low in tear strength by reason of the occurrence of significant molecular orientation during the sheet-forming process. Besides, PVDF is not superior to other fluorohydrocarbon resins in low temperature characteristics represented by impact resistance and brittle point temperature and, therefore, is not suited to uses wherein good low temperature characteristics are strongly required. Though various proposals have been made for decreasing the crystallinity of PVDF and affording elasticity to PVDF by either of the above described copolymerization method or blending method, the above described problems and difficulties have not yet been solved or eliminated. For example, it has been reported that polymethyl acrylate is high in mutual solubility with PVDF so that a PVDF base resin composition which is elastic and excellent in drawability can be obtained by blending PVDF with polymethyl acrylate. In such a resin composition, however, it is inevitable that the content of fluorine considerably decreases as the elasticity is improved by the addition of polymethyl acrylate that does not contain fluorine in its molecular chain, and therefore the resin composition becomes degraded in some important properties such as resistance to chemicals and weather resistance.
Also it has been considered to blend PVDF with fluoroelastomer that is a fluorine-containing polymer low in glass transition temperature T.sub.g. Actually, however, the blending does not give good results because fluoroelastomers are generally low in mutual solubility with PVDF and therefore the blend locally remains in heterogeneously mixed state and becomes inferior in dynamic properties, or it is difficult to blend a sufficient amount of fluoroelastomer with PVDF.